Move the selection of CONFIG_FPU to each CPU type configuration.

Currently for m68k we have a global set of CONFIG_FPU based on if CONFIG_MMU
is enabled or not. There is at least one CPU family we support (m5441x)
that has an MMU but has no FPU hardware. So we need to be able to have
CONFIG_MMU set and CONFIG_FPU not set.

Whether we build for a CPU with MMU enabled or not doesn't change the
fact that it has FPU hardware support. Our current non-MMU builds have
never had CONIG_FPU enabled - and in fact the kernel will not compile
with that set and CONFIG_MMU not set at the moment. It is easy enough
to fix this - but it would involve a structure change to sigcontext.h,
and that is a user space exported header (so ABI change).

This change makes no configuration visible changes, and all configs
end up with the same configuration settings as before.

This change based on changes and discussion from Yannick Gicquel
<yannick.gicquel@open.eurogiciel.org>.
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>